1. Field of the Invention
The present invention is in the field of a gun assembly used for underground jet perforating while exploring for oil and/or gas and extracting the same from underground. More particularly, the present invention is directed to a gun assembly containing a plurality of shaped charges, the explosion of which is triggered by resistive blasting caps and which can be detonated in sequential order, and to an electronic module that renders the sequential detonation possible.
2. Background Art
It has been common practice for a long time in the oil and gas production industry to perforate the wall of the oil or gas well casing at locations where entry of oil or gas from the surrounding formation into the casing is desired. The prior art has created shaped explosive charges for this purpose. The charges are detonated by passing current through a blasting cap that ignites the charge through a detonating fuse. The term "shaped charge" in this regard is well understood in the art, and denotes an explosive charge specifically adapted for the purpose of creating certain desired size holes in the casing, and a desired amount of penetration into the surrounding formation.
In connection with the foregoing, a number of shaped charges are assembled in a "gun assembly" which is lowered into the well casing on a wireline including a logging cable. Spaces in the gun assembly which contain the individual charges are separated from one another by baffle plates that are usually not destroyed when the charge below the baffle plate is detonated. Sometimes, it is satisfactory to simultaneously detonate all shaped charges in the gun assembly. More frequently, however, it is desired to detonate the shaped charges sequentially, one by one, usually starting with the shaped charge that is located at the bottom of the gun assembly. This is commonly called "select fire" in the trade, and the prior art has developed several methods for accomplishing such "select fire" detonation. One such method utilizes a rotary switch operated at the surface with which the several charges can be detonated. This method, however, has its disadvantages, primarily in that the number of charges which can be detonated in this manner is limited.
Another prior art method, that is presently believed to be the most pertinent background to the present invention permits sequential "select fire" detonation of the charges starting at the bottom of the gun assembly, by sequentially applying direct current (d. c.) voltage of alternating polarity to the logging cable from the surface. In accordance with this method, the logging cable is electrically connected through a diode to the blasting cap attached to the charge on the bottom of the gun assembly, and this blasting cap is grounded. All other blasting caps attached to the other charges above the bottom charge are not grounded. Instead they are electrically connected to the diode and a dart which is mounted through an insulating gasket to the baffle plate. The diode is also connected to the logging cable. The dart is a device, well known in the trade, that seals the baffle from the portion of the gun assembly below, when the charge immediately below the dart has been detonated. In the process, by breaking through the insulating silicone gasket, the dart also becomes electrically grounded and thereby it grounds the blasting cap to which the dart is connected.
The diodes are mounted into an electronic module that also contains a small resistor (approximately 5 .OMEGA.) which is placed in series with the logging cable. Thus, each diode is connected to the logging cable and to the blasting cap, but except for the blasting cap on the bottom of the gun assembly, the rest of the caps are grounded only after the charge immediately below the dart has been detonated. The diodes are mounted with sequentially reversed polarity, so that for example, the diode on the bottom of the assembly permits current to pass through when negative voltage is applied on the surface, the diode above that passes current on positive voltage, the one above that again on negative voltage, and so on. Therefore, when negative voltage is applied to the logging cable on the surface, the diode on the bottom allows current to pass through the blasting cap which is grounded, and the charge on the bottom is detonated. The charges above are not detonated in this first application of negative voltage because the respective blasting caps are not grounded. Nevertheless, current can flow through to the bottom diode and blasting cap, because the logging cable, including the resistors built into the modules, represent a continuous electrical path. After the first charge has detonated, the dart in the baffle above breaks through its silicon gasket, seals the baffle into which it is mounted, and electrically grounds the blasting cap attached to it. This blasting cap receives current through the corresponding diode when positive voltage is applied on the surface. Thus, in accordance with this method, a series of explosive charges built into the gun assembly can be sequentially detonated, starting with the charge on the bottom. The resistor incorporated in each electronic module in series with the logging cable, serves to allow current to flow through to the successive caps on the bottom, even if the wire below such caps is grounded.
Blasting caps are usually manufactured to activate when approximately 0.25 to 0.8 amper current flows through them. More specifically, in accordance with practice in the art, blasting cap specifications usually state that the cap will not be activated by current less than approximately 0.25 A, but are certain to be activated with 0.8 A current. The blasting caps, which until relatively recently have been used in the prior art, had very low resistance so that the 0.3 to 0.8 A current could be accomplished by applying low voltage. Relatively recently, for safety reasons, however, blasting caps have been made with higher internal resistance, so that they can be activated only with higher voltage (approximately 25 to 100 Volts). Such blasting caps are called "resistive caps" and usually have internal resistance approximately in the 50 to 120 .OMEGA. range. The just described prior art method of select fire gun assembly is not well suitable for use with resistive caps. A primary reason for this is that for the cap of such resistance to draw enough current to be activated, the parallel disposed resistor would have to have much greater resistance. However, that would require very high voltage in order to send sufficient current through the combined resistance of the resistors which are disposed between the voltage source and the blasting to be activated. Also in accordance with this prior art method of select fire, if detonation of a charge left the wire below intact and grounded, then high resistance parallel with the cap would be needed for that cap to draw enough current for activation. Consequently, use of this method is not practical when the cap itself has resistance in approximately 50 to 120 B range, or higher.
In light of the foregoing, there is a need in the art for a gun assembly containing an electronic module that permits select fire detonation of charges for jet perforation which works well with resistive blasting caps. The present invention provides such a gun assembly and electronic module.